Chlamydia trachomatis infections have a prevalence of 2-15% in adolescents/young adults in the USA and Western Europe in spite of public health efforts and effective antibiotic therapy. It is widely accepted that development of a Chlamydia vaccine is necessary reduce its prevalence. A critical component of vaccine development is identification of a surrogate biomarker for protective immunity to enable screening of candidate vaccines. Currently there are no practicable biomarkers for Chlamydia protective immunity. The Johnson laboratory has identified a second CD4 T cell-mediated mechanism singly-sufficient for clearing Chlamydia from the genital tract that is dependent on Plac8. The Brunham laboratory has demonstrated that vaccine-generated protective immunity against Chlamydia genital tract infections requires a multifunctional Th1 response. The two laboratories approaching protective immunity from different perspectives have converged on working model for protective immunity that has major potential implications for vaccine development. Supported by published and unpublished data they postulate that the foundation of a protective C. trachomatis vaccine will be generation of a multifunctional CD4Plac8 anti-Chlamydia response, and that multifunctional CD4Plac8 T cell responses will serve as practicable biomarkers for protective immunity and vaccine efficacy. To test that hypothesis and investigate the underlying immunobiology they propose the following specific aims: Specific aim #1- to investigate the role of the Plac8-dependent clearance mechanism in vaccine-generated protective immunity. This aim will utilize the Brunham laboratory's published protective murine PmpG vaccine, and existing knockout mice. Specific aim #2- to investigate the effector mechanism associated with Plac8-dependent immunity. This aim will utilize the Johnson lab's published Chlamydia-specific Plac8pos and Plac8neg CD4 T cell clones, and Plac8 knockout reproductive tract epithelial lines. Specific aim #3- to investigate the frequency and distribution of multifunctional CD4Plac8 T cells in mice and humans, in the absence and presence of Chlamydia infections. This is novel translational research.